Page 11 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

FIGURE 7
byproduct being H 2 , thus providing attractive “green” synthetic
TABLE 1. Mild Hydrogenation of Esters to Alcohols Catalyzed by the
PNN Pincer Complex 11 methods. Following our report, other reports on amide forma-
tion from alcohols and amines appeared. 37 Interestingly, the
metalligand cooperation concept was applied in nonpincer
complexes for the coupling of alcohols with water, alcohols and
amines; however, sacrificial hydrogen acceptors were used. 38

Catalytic Hydrogenation of Esters, Amides,
and Ketones
Complex 11 also catalyzes the hydrogenolysis of esters to
9
alcohols under 5 atm of H 2 . The reaction is general and not
limited to activated esters (Table 1). It proceeds under relatively
mild, neutral conditions, providing a “green”, mild pathway for
the synthesis of alcohols from esters. The reaction is sensitive to
steric hindrance of the esters. Ester hydrogenation is still rare. 39
A possible mechanism of the reaction is outlined in
Figure 8. Addition of H 2 to the dearomatized PNN complex
11 leads to the trans-dihydride complex 12, which can
undergo decoordination of the amine “arm” to provide a
site for ester coordination. Hydride transfer to the ester
carbonyl and subsequent amine arm coordination and
dearomatization of the pyridine core regenerates complex
11 and eliminates a hemiacetal, which is in equilibrium with
the aldehyde in solution. The aldehyde is then hydroge-
nated to the corresponding alcohol via a similar cycle. Note
whereas with the PNP complex 8, the anticipated water that direct attack of the trans dihydride on the ester carbonyl
elimination takes place to provide imines. 35 group, without prior coordination, is also possible.
The striking difference in the catalytic activity of the PNN and Under mild hydrogen pressure (10 atm), it is possible to
PNP complexes can be rationalized as follows. In the case of the catalyze the unprecedented selective hydrogenolysis of
PNN complex, bearing the hemilabile amine arm, the coordi- amides to the corresponding alcohols and amines, involving
nated aldehyde is attacked by the primary amine, forming a CN cleavage. The bipyridine-based dearomatized PNN
quaternary ammonium intermediate, followed by intramolecu- complex 39 shows higher activity than 11 (Scheme 21). 40
larprotontransfertothedearomatizedphosphinearm(Figure7), Nozaki reported Ir(III)PNP pincer catalyzed efficient hy-
while in the case of the PNP complex attack of the aldehyde by drogenation of CO 2 to formate salts, also suggested to
theaminetakesplaceafteritisreleasedintosolution,generating involve dearomatization of the pincer ligand. 41
a free hemiaminal intermediate which liberates water. A similar Very recently, we have reported facile hydrogenation
conclusion was reached in another study. 37d of ketones to the corresponding secondary alcohols under
These direct transformations of alcohols to esters, amides, mild hydrogen pressure (4.1 atm) catalyzed by the iron com-
and imines proceed in high yields and high turnover numbers plex 40,thatoperates bymetalligand cooperation via
under neutral conditions with no generation of waste, the only ligand dearomatizationaromatization (Scheme 22). 42a

598 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 588–602 ’ 2011 ’ Vol. 44, No. 8